Recent oil spills have focussed attention on the importance of efficient clean-up methods. Leaks in pipelines or oil-storage units or from oil tankers can cause surface and buried spills. Also, after a surface spill on a shoreline, wave action can result in burying the contaminated areas.
After an oil spill, clean-up operations are facilitated by first determining the location and depth of buried spills and the depth of surface spills. Once the location and depth of the spill is known, the resources and effort needed for clean-up operations can be allocated. For example, certain types of clean-up operations, such as microbial treatment, are more appropriate for shallower saturations than for deep ones.
One prior method for determining the depth of crude oil is the excavation of a test pit, which permits the saturation depth to be seen. However, this method is time consuming and requires expensive earth moving equipment at the site.
A second method is the use of electrical resistivity measurements. These methods measure the difference between the electrical resistivity of the contaminated and the uncontaminated areas. However, this method is prone to error, especially in the case of oil tanker spills. The oil in a sandy shoreline will displace air but not water, and because the shoreline is likely to be wet, the effect of the oil is small and difficult to measure.
A third method is ground penetrating radar measurements. A problem with this method is that the detectible difference between the contaminated and the uncontaminated areas is low. Thus, the contaminated area's interface does not produce a good reflector for electromagnetic waves.
A fourth method is seismic measurements. Problems with this method include lack of resolution and a poor reflecting interface. The attenuation of the seismic waves is high, especially in shorelines, which tend to consist of unconsolidated sand and gravel.
In light of the above problems with the prior art, a need exists for a reliable and accurate method for measuring the depth of crude oil in a subsurface.